1. Field of the Invention
This invention relates to image processing wherein image data produced by a host computer is quantized to multi value data prior to using it in an output apparatus such as a display apparatus or a printer. More specifically, the invention relates to novel methods and apparatus for image processing wherein image data is produced at a higher resolution than that of the original image data from the host computer.
2. Description of the Related Art
Recent developments in image processing have permitted printers and display apparatus to form images at very high resolutions; which in some cases, are higher than that used in a host computers.
By way of background, reference may be made to FIG. 1 of the accompanying drawings. This figure shows a print head 1 and associated parts of a known ink jet printer. Print heads in such printers typically have resolutions in the range of 600 dpi (dots per inch).
The print head 1, which has a resolution of 600 dpi, is provided with 128 ink discharge openings which are spaced apart from each other by 42.3 microns. These discharge openings are arranged so that they eject ink onto a recording paper 2 as the print head 1 scans across the recording paper 2 in a sub-scanning direction. Between successive scans of the print head 1 in the sub-scanning direction, the paper 2 is driven in a forward direction by a roll 3.
Each of the 128 discharge openings in the print head 1 has associated ink path through which liquid ink passes from a supply (not shown). Individual heaters are provided along each of these ink paths within the print head 1. These heaters generate thermal energy in response to electrical pulses received from a driver controller. The thermal energy causes film boiling of the liquid ink in the associated ink path; and this in turn causes a bubble to be generated which forces a droplet of ink out of the respective print head discharge opening and onto the recording paper 2. The pulses from the driver controller are coordinated with the scanning movement of the print head 1 so that the droplets of ink will be discharged at desired locations on the recording paper. The frequency of heater driving signals, that is, the discharge frequency of ink droplets from the print head, is 10 KHz.
The size of the print head discharge openings is set such that the ink droplets will appear at a predetermined density upon landing on the recording paper 2. This density, known as "Optical Density" (O.D.), has a value of b 1.4 when one ink droplet lands on a 42.3 micron square pixel.
A carriage 4 on which the print head 1 is mounted is supported by a pair of guide shafts 5A and 5B which extend in the direction of sub-scan, that is, across the recording paper 2; thereby to allow the print head 1 to scan over the recording paper in the sub-scan direction. The carriage 4 is connected to a belt which extends over pulleys (not shown) near the ends of the guide shafts 5. The pulleys, which are driven by a motor, cause the belt to move and pull the carriage 4 and the print head 1 along in the sub-scan direction.
A flexible ink tube 6 connects the print head 1 to a stationary ink supply tank (not shown) so that liquid ink from the tank can be supplied continuously to the print head as it moves along in its scanning direction.
A flexible cable 7 connects the moveable print head 1 to a stationary head driver circuit (not shown). Electrical pulses from the head driver circuit are supplied via the cable 7 to the print head 1 as it scans in the sub-scan direction. These electrical pulses are applied to the heaters in the print head at a timing based on information from the print head driver; and they cause the heaters to generate thermal energy to eject ink droplets as described above. The timing of the electrical pulses is controlled by the print head driver so that the ink droplets land at predetermined locations on the recording paper 2.
The longitudinal axis of the platen roller 3 extends parallel to the guide shafts 5A and 5B; and as it turns about its axis, the platen roller drives the recording paper 2 in a direction perpendicular to the sub-scanning direction. The platen roller 3 also provides a solid back support for the recording paper 2 in the region thereof where the ink droplets ejected from the print head 1 land. As will be appreciated, the movement of the print head in the sub-scan direction and the movement of the recording paper 2 in the perpendicular direction, provide coordinates which define precisely where ink droplets from the print head 1 will land on the recording paper 2. By proper control of these coordinates and by synchronizing them with the pulses which are supplied to the print head heaters, ink drops can be directed at appropriate locations on the recording paper 2 to form a desired image.
FIG. 2 is a block diagram showing a known control structure for the ink jet apparatus as shown in FIG. 1. As shown in FIG. 2, a main controller 112, which includes a central processing unit (CPU), receives image data from a host computer 111 and supplies the data to a frame memory 113 where it is stored. The image data is arranged according to pixels which correspond to specific locations on the recording sheet 2 (FIG. 1). The image data for each pixel comprises 8 binary bits, which provides the pixel with a possibility of 256 tones. The main controller 112 causes this pixel tone data to be transferred from the frame memory 113 to a pseudo half toning process section 114, and from there to a driving RAM (random access memory) 115. The pseudo half toning process section 114 converts the 8-bit tone data of each pixel into 1-bit data. This pseudo half toning processing may be carried out according to any of several well known algorithms, some of which are known as the "dither method", the "error diffusion method", etc.
A driver controller 116, in response to a control signal from the main controller 112, reads out driving data which is stored in a driving data RAM 115. This data, which corresponds to each discharge opening of the print head 1, is supplied to a head driver 117 and controls its driving timing.
In the above described structure, the main controller 112 operates through the driver controller 116 and the head driver 117 to control ink discharge from the print head 1 (118 in FIG. 2). The main controller 112 also operates through a carriage motor driver 119 to control movement of a carriage drive motor 121; and it operates through a paper feed motor driver 120 to control movement of a paper feed motor 122. As a result, characters and graphic images corresponding to input image data are printed on the recording paper 2 and paper feed motor driver 120 and paper feed motor 122, together constituting a transporter, transport the recording paper 2.
The print head in the ink jet printer of FIG. 1 has a resolution of 600 dpi; which makes it possible to print at a relatively high resolution. However, if the image data from the host computer 111 has a resolution of only about 300 to 400 dpi and is processed with the system of FIG. 2, the high print resolution capability of the printer cannot be realized.
When a print head which has a resolution of 600 dpi prints from 300 dpi image data using the system of FIG. 2, each data bit will correspond to four dots which occupy a 2.times.2 matrix. As a result, the printed image is reduced from a possible five tones to only two tones. On the other hand if, by means of resolution conversion, 600 dpi image data is supplied to the print head, it is possible to obtain a high tone output from the printer. However, the time required to transfer data from the host computer 111 to the main controller 112 in such case is about 4 times that which would be required for 300 dpi resolution printing. As the conversion magnification increases, the required data transfer time also increases; and therefore this technique is not practical for high resolution print heads.
Though the above explanation concerns an image processor in a print system using a print head, it will be appreciated that this same situation will occur in the case of a display system which forms a visible image on a liquid crystal device or on a cathode ray tube (CRT).